Thioresistant catalyst for methane production

ABSTRACT

A method of methane production with a thioresistant catalyst wherein a mixture comprising in particular carbon monoxide, hydrogen and sulphur compounds is contacted with a thioresistant catalyst comprising a metal selected from the group comprising molybdenum, vanadium or tungsten and possible cobalt and/or nickel, this catalyst being deposited onto a cerium oxide support, the reaction being performed at a temperature lying between about 250° C. and 650° C. and at a pressure lying between about 5 bars and 140 bars. This method is useful to carry out the effective methane synthesis with a much improved selectivity in favor of methane.

This is a division of application Ser. No. 077,694, filed July 24, 1987,now U.S. Pat. No. 4,774,261, which is a continuation of application Ser.No. 812,179, filed Dec. 23, 1985, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates essentially to a method of production ofmethane or of a methane-containing gas mixture by means of a sulphurresistant or thio-resistant catalyst.

It is also directed to the catalyst for carrying out this method.

It is already known to perform the synthesis of methane from a gasmixture comprising among other components carbon monoxide, hydrogen,water and sulphur compounds and said gas mixture may in particular beobtained through coal gasification and be contacted with a catalystlikely to activate the synthesis reaction.

For that purpose, various catalysts have been proposed but it has beenfound that they quickly become spent or exhausted or deactivated in viewof the presence of sulphur compounds.

Thus, catalysts for the methanation reaction which which are based forinstance on molybdenum and/or nickel deposited onto an alumina supporthave already been proposed but such catalysts are far from being verysatisfactory with regard to their activity and their selectivity infavour of methane production.

SUMMARY OF THE INVENTION

Therefore, the object of the present invention is to cope with suchinconveniences by providing a process of methane production with theassistance of thioresistant catalysts which give outstanding resultswith respect to the carbon monoxide conversion as well as to theselectivity in terms of produced methane and of the stability with timeof the performances of the catalyst.

For that purpose, the subject matter of the invention is a process ofproduction of methane or of a mixture comprising methane, from aninitial mixture comprising in particular carbon monoxide, hydrogen andsulphur compounds, characterized in that said initial mixture iscontacted with a thioresistant catalyst comprising a metal selected fromthe group comprising molybdenum, vanadium or tungsten and possibly anactivity promoter consisting of cobalt and/or nickel, which catalyst isdeposited onto a cerium oxide support at a temperature lying betweenabout 250° C. and 650° C. and at a pressure lying between about 5 barsand 140 bars.

As will be seen later, the selectivity in terms of methane achieved bythis method is much higher than that obtained with prior art methodsowing to the essential fact that according to the present invention acatalyst deposited onto cerium oxide is used.

According to another characterizing feature of the process according tothe invention, the reaction is accomplished at a space velocity (V/V/hr)lying between about 100 hours⁻¹ and 15,000 hours⁻¹ and with ahydrogen/carbon monoxide molar ratio equal to at least 0.3 andpreferably equal to 1.

According to a preferred embodiment, the reaction is performed at aspace velocity equal to 4,750 hours⁻¹ and with a hydrogen-to-carbonmonoxide molar ratio equal to 1, at a pressure equal to 30 bars and at atemperature lying between 300° C. and 600° C. and preferably equal to500° C.

The invention is also directed to a catalyst for carrying out theprocess meeting either one of the above characteristics, this catalystbeing characterized in that it is represented by the formula X/CeO₂,X/Co/CeO₂, X/Ni/CeO₂ or X/Co/Ni/CeO₂ wherein X stands for molybdenum,vanadium or tungsten and in that it is defined by the followingcharacteristics:

a BET specific area of the CeO₂ support above 10 m² /g,

a total pore volume lying between about 0.15 cm³ /g and 0.5 cm³ /g,

a packed filling density lying between about 0.5 and 2.5,

an atomic ratio of said X metal to cerium lying between about 1/50 and1/4, and

an atomic ratio of the activity promoter metal (cobalt and/or nickel) toX metal lying between 0 and 1.

According to a preferred embodiment, the BET specific area of theabove-mentioned catalyst is equal to 50 m² /g, its total pore volume islying between 0.3 cm³ /g and 0.4 cm³ /g, the packed filling density islying between 1 and 2, the X/cerium atomic ratio is lying between 1/20and 1/7, and the promoter-to-X metal atomic ratio is lying between 0.1and 0.5.

This catalyst may be prepared by conventional techniques and inparticular through impregnation of the support with solutions of theprecursors of those metals which are desirably added.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Further characterizing features and advantages of the invention willbetter appear in the following example which should not be construed aslimiting the present invention.

Three catalysts deposited onto cerium oxide have at first been prepared,namely: a molybdenum/cerium oxide catalyst, a cobalt/molybdenum/ceriumoxide catalyst and a nickel/molybdenum/cerium oxide catalyst.

The Mo/CeO₂ catalyst had a BET specific area of 42 m² /g, a total porevolume of 0.14 cm³ /g, a packed filling density of 1.83 and a molybdenumcontent of 3.6% by weight.

The Co/Mo/CeO₂ catalyst had a BET specific area of 39 m² /g, awater-porous volume of 0.11 cm³ /g and a packed filling density of 1.92.

At last, the Ni/Mo/CeO₂ catalyst had a BET specific area of 37 m² /g, awater-porous volume of 0.11 cm³ /g and a packed filling density of 1.95.

Then, a catalytic test for comparing the performances of the threeabove-mentioned catalysts to known alumina or aluminum based catalystsand complying with the following formulae: Mo/Al₂ O₃, Co/Mo/Al₂ O₃,Ni/Mo/Al₂ O₃ and Ce/Mo/Al was carried out.

This catalytic test which had the purpose of comparing the performancesof all the above-mentioned catalysts in terms of activity andselectivity in the preparation of methane consisted in contacting allthese catalysts with a mixture of reagents comprising in a first stage(conditions of table 1) 37.25% by volume of CO (carbon monoxide), 37.25%by volume of H₂, 25% by volume of H₂ O and 0.5% by volume of H₂ S and,in a second stage (conditions of table 2) 49.75% by volume of CO, 49.75%by volume of H₂ and 0.5% by volume of H₂ S.

The reaction was performed at a space velocity of 4,750 hr⁻¹ under apressure of 30 bars and a temperature of 500° C. after the catalysts hadundergone a presulphuration treatment at 350° C. for 6 hours under astream of 10 liters/hour of a mixture of 1.3% of H₂ S in hydrogen.

After the separation of water by a condenser, the issuing gases wereanalyzed through gas-chromatography which made it possible to proportionin particular CO, CO₂, CH₄, C₂ H₆ and C₃ H₈.

This analysis has allowed computation of on the one hand the carbonmonoxide conversion rate (%) defined by the ratio: ##EQU1## and, on theother hand, the selectivity in terms of CH₄ (%) defined by the ratio:##EQU2##

Tables 1 and 2 hereinafter show the results obtained with all thecatalysts previously described before and after an ageing correspondingto 100 hours of operation or work of the catalyst.

                  TABLE 1                                                         ______________________________________                                                     Before ageing                                                                            After ageing                                          Catalysts      t.sub.CO %                                                                            S.sub.CH.sbsb.4 %                                                                      t.sub.CO %                                                                          S.sub.CH.sbsb.4 %                       ______________________________________                                        Prior art                                                                            Mo/Al.sub.2 O.sub.3                                                                       61.7    15.2   57.4  12.1                                         Co/Mo/Al.sub.2 O.sub.3                                                                    62.6    24.8   61.6  19.1                                         Ni/Mo/Al.sub.2 O.sub.3                                                                    66.7    22.3   65    18.5                                         Ce/Mo/Al    71      21.9   64.8  20.8                                  Invention                                                                            Mo/CeO.sub.2                                                                              80.4    30.4   77    28                                           Co/Mo/CeO.sub.2                                                                           87.3    36.7   81.1  32.7                                         Ni/Mo/CeO.sub.2                                                                           88.1    35.3   81.5  31.9                                  ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                                     Before ageing                                                                            After ageing                                          Catalysts      t.sub.CO %                                                                            S.sub.CH.sbsb.4 %                                                                      t.sub.CO %                                                                          S.sub.CH.sbsb.4 %                       ______________________________________                                        Prior art                                                                            Mo/Al.sub.2 O.sub.3                                                                       78.3    49.9   69.4  50.0                                         Co/Mo/Al.sub.2 O.sub.3                                                                    79.1    50.0   70.9  49.5                                         Ni/Mo/Al.sub.2 O.sub.3                                                                    78.8    49.6   71.9  49.8                                  Invention                                                                            Mo/CeO.sub.2                                                                              85.8    49.7   78.2  49.8                                         Co/Mo/CeO.sub.2                                                                           87.1    50.0   78.7  50.0                                         Ni/Mo/CeO.sub.2                                                                           86.6    48.0   79.0  47.3                                  ______________________________________                                    

The results given in table 1 clearly substantiate the superiority of thecatalysts according to the invention over the prior art catalysts as tothe carbon monoxide conversion rate, the selectivity in terms ofproduced methane and the stability with time of the performance.

More specifically, it should be pointed out that the Mo/CeO₂ catalyst isnot only superior to the three first prior art catalysts containing nocerium oxide, but is also superior to the fourth prior art catalystmentioned in table 1 and which is based on cerium, molybdenum andaluminum.

This superiority is also clearly seen with respect to the Co/Mo/CeO₂ andNi/Mo/CeO₂ catalysts if the same are compared to the prior art catalystsand in particular to the Co or Ni catalysts with molybdenum depositedonto alumina. It is indeed seen here that with these two catalysts, aselectivity in methane lying between 32% and 36% and CO conversion ratesabove 80% are achieved.

The results given in table 2 (absence of water in the starting mixture)show that CO conversion rates above 85% and definitely better than thoseobtained with the prior art catalysts are achieved. The results are alsobetter with respect to the stability with time of the performances.

The selectivity in terms of produced methane is substantially the sameas that of the prior art catalysts and in any case lies at thethermodynamic limit.

There has accordingly been provided according to the invention amethanation process which allows recovery of substantial amounts ofmethane by using a very stable cerium oxide-based catalyst effectivelyresistant to sulphur compounds such as H₂ S, COS, CS₂, CH₃ S, etc. . . .and the content of which may exceed 4 molar percent of sulphur.

It should be understood that the invention is not at all limited to theembodiment or the example described hereinabove.

Thus, by replacing molybdenum with vanadium or tungsten in the formulaeof the catalysts according to the principle of the invention, resultsand performances close to those described hereinbefore are obtained.Likewise, the joint use of two activity promoter metals (cobalt andnicke) instead of a single one would lead to a catalyst quite active infavour of methane production.

The invention therefore comprises all the technical equivalents of themeans described as well as their combinations if same are carried outaccording to its substance.

What is claimed is:
 1. A catalyst for the production of methane or amethane-containing gas mixture from a mixture of carbon monoxide,hydrogen and sulphur compounds, said catalyst being of the formulaX/CeO₂, X/Co/CeO₂, X/Ni/CeO₂ or X/Co/Ni/CeO₂ wherein X stands formolybdenum, vanadium or tungsten and said catalyst being defined by thefollowing characteristics:a BET specific area of the CeO₂ support above10 m² /g; a total pore volume between about 0.15 cm³ /g and 0.5 cm³ /g;a packed filling density between about 0.5 and 0.25; an atomic ratio ofsaid X metal to cerium between about 1/50 and 1/4; and an atomic ratioof the activity promoter metal (Co and/or Ni) to the X metal between 0and
 1. 2. A catalyst according to claim 1, wherein said BET specificarea is equal to 50 m² /g, said total pore volume is between 0.3 cm³ /gand 0.4 cm³ /g, the packed filling density is between 1 and 2, theX/cerium atomic ratio is between 1/20 and 1/7 and the promoter/X metalatomic ratio is between 0.1 and 0.5.